Computer-based navigation systems for use on land have become available in a variety of forms and provide a variety of useful features.
One exemplary type of navigation system uses (1) a detailed data set of one or more geographic areas or regions, (2) a navigation application program, (3) appropriate computer hardware, such as a microprocessor, memory, and storage, and optionally, (4) a positioning system. The detailed geographic data set portion of the navigation system is in the form of one or more detailed, organized data files or databases. The detailed geographic data set may include information about the positions of roads and intersections in or related to one or more specific geographic regional areas, and may also include information about one-way streets, turn restrictions, street addresses, alternative routes, hotels, restaurants, museums, stadiums, offices, automobile dealerships, auto repair shops, etc.
The positioning system may employ any of several well-known technologies to determine or approximate one's physical location in a geographic regional area. For example, the positioning system may employ a GPS-type system (global positioning system), a "dead reckoning"-type system, or combinations of these, or other systems, all of which are well-known in the art.
The navigation application program portion of the navigation system is a software program that uses the detailed geographic data set and the positioning system (when employed). The navigation application program may provide the user with a graphical display (e.g. a "map") of the user's specific location in the geographic area. In addition, the navigation application program may also provide the user with specific directions to locations in the geographic area from wherever the user is located.
Some navigation systems combine the navigation application program, geographic data set, and optionally, the positioning system in a single unit. Such single unit systems can be installed in vehicles or carried by persons. Alternatively, navigation application programs and geographic datasets may be provided as software products that are sold or licensed to users to load in their own personal computers. In further alternatives, the navigation system may be centrally or regionally located and accessible to multiple users on an "as needed" basis, or alternatively, on-line via a network or communications link. Personal computer-based systems may be stand-alone systems or may utilize a communication link to a central or regional or distributed system. Also, users may access a navigation system over an online service such as the Internet, or over private dial-up services, such as CompuServe, Prodigy, and America Online. In-vehicle navigation systems may use wireless communication connections. Navigation systems may also be used by operators of vehicle fleets such as trucking companies, package delivery services, and so on. Navigation systems may also be used by entities concerned with traffic control or traffic monitoring.
Computer-based navigation systems hold the promise of providing high levels of navigation assistance to users. Navigation systems can provide detailed instructions for traveling to desired destinations, thereby reducing travel times and expenses. Navigation systems also can provide enhanced navigation features such as helping commuters and travelers avoid construction delays and finding the quickest routes to desired destinations. Navigation systems can also be used to incorporate real-time traffic information.
In order to provide these useful and enhanced features in a navigation system, there is a need to gather and organize comprehensive, detailed, reliable, and up-to-date data about geographical regions and areas. There is also a need to continuously update the geographic data since many data can rapidly become out-of-date. Presently, the collection of such geographic data and the provision of such data in a computer-usable format are provided by Navigation Technologies of Sunnyvale, Calif.
One known way to generate a digital database of road geometry in a geographic region is to obtain an aerial photograph of the geographic region. A technician, using a digitizing pad or other suitable input device, selects points from the aerial photograph to create "nodes." Two nodes are connected by a segment where the segment represents the portion of the road between two nodes. Nodes may be positioned at intersections or at the ends of a roadway, for example. A disadvantage with this method is the relatively high cost of obtaining aerial photographs especially in geographic regions where there is not much road geometry. A further disadvantage is the cost of labor and equipment to digitize the aerial photographs.
Alternatively, under certain limited circumstances, as explained below, the Global Positioning System (GPS) may be used to gather such geographic data. By way of background, the Global Positioning System was developed by the U.S. Government in order for the military to have a precise form of worldwide positioning for maneuvering, navigation, targeting, and so on. The GPS is now used for many non-military purposes including navigation on land, sea, and in the air.
A GPS receiver acquires GPS signals sent from a plurality (e.g., "a constellation") of satellites. GPS uses the satellites in space as reference points for locations on earth. The basis of GPS is triangulation from a plurality of satellites. By accurately measuring the receiver's distance from each satellite in the constellation, one can triangulate the receiver's position anywhere on earth. To triangulate, a GPS receiver located on earth measures the distance of the receiver from each of the satellites of a constellation using the travel times of radio signals from each of the satellites.
In order to prevent civilians from using the same data to set artillery or home an unmanned ammunition to a target with optimum precision, the government intentionally induces an error (called selective availability (SA)) ranging from 0 to about 100 meters into the signals transmitted by the satellites. Military receivers are not affected by SA because they have access to a decryption key to remove the SA errors.
In order to correct for the error induced by SA, a process called differential correction was developed. In general terms differential correction or DGPS is based upon the principal that if standard GPS signals indicate where one is within 100 meters and one knows there is a random, dynamic error induced in the GPS signals received, the first step to solving this algebraic unknown is to establish a base or a known position. For example if a surveyed site has the coordinates (N. 39 00' 00" W. 120 00' 00") and a standard GPS signal is recorded at that surveyed site and has the coordinates (N. 39 00' 02" W 120 00' 00"), a difference of 2" (about 202 feet or 67 meters) to the north exists between the true location and the detected location. If one were receiving the same GPS signal at the same time at a different relatively nearby site, a correction of the coordinates 67 meters to the south removes the induced error and provides a corrected geographic position.
An automobile equipped with a GPS receiver and antenna can be used to collect geographic data in a geographic region. As the vehicle is driven on a roadway in the geographic region, the GPS receiver is operated. The GPS signals are stored as standard GPS data in memory and later transmitted to a remote site where the standard GPS data is further processed. Correction data may be provided by a reference GPS receiver located at a known, surveyed site, i.e., a base station. The roving GPS receiver, i.e., the GPS receiver in the vehicle, records all of the GPS signals it receives and the time each signal is received. The base station calculates and stores correction data for the GPS signals the base station receives and creates a correction data file. The standard GPS data are post processed with the correction data to obtain corrected GPS data. There are several disadvantages, however, with this method. These include the cost of supplying and maintaining a base station, and if the base station is provided by a third party, a compromise in reliability may be introduced since the third party may not be acquiring corrections for GPS signals at the same periods of time, or intervals of time within those periods, as the roving receiver. Also the distance that the roving receiver may be from the base station is limited to about 300 miles. At present time, still less than about 30% of the U.S. is covered within the range of base stations. Also, this method requires post processing of the data gathered by the roving receiver which does not allow for precise positioning in real time and introduces delay in the availability of the geographic information gathered by the GPS receiver.
Thus, it is desirable to provide a system and method for acquiring data for creating a digital database of road geometry in a geographic region wherein the GPS signals may be differentially corrected. It is also desirable to provide a system and method that provides more reliability and a greater area of coverage than the method relying upon base stations.